The invention relates generally to methods and reactors for cracking hydrocarbon. More specifically, the invention relates to methods and reactors for cracking hydrocarbon, in which the build-up of coke deposits are undesirable.
In the petrochemical industry, hydrocarbons such as ethane, propane, butane and naphtha are cracked in reactors, in the presence of from about 30 to 70 weight percentage of steam, at temperatures of from about 700° C. to 870° C. in order to produce light olefins such as ethylene and propylene. Sometimes, hydrocarbons such as bottoms from atmospheric and vacuum distillation of crude oil are cracked in reactors at a temperature in a range from about 480° C. to about 600° C. in the presence of about 1 wt % to about 2 wt % steam.
During hydrocarbon cracking processes, the build-up of carbonaceous deposits (i.e. coke deposits) usually happens on inner surfaces of reactor components, for instance, inner radiant tube surfaces of furnace equipment. The inner radiant tube surfaces become gradually coated with a layer of coke which raises the radiant tube metal temperature (TMT) and increases the temperature drop through radiant coils. In addition, coke build-up adversely affects the physical characteristics of the reactor components, such as the radiant tubes, by deteriorating mechanical properties such as stress rupture, thermal fatigue, and ductility.
In order to decoke reactor components, the reactor must be periodically shut down. Typically, the decoking is carried out by combustion of the coke deposits with steam/air at temperatures of up to 1000° C. Such decoking operations are required approximately every 10 to 80 days, depending on the operation mode, types of hydrocarbons and hydrocarbons throughput, and result in production loss since hydrocarbons feeding must be stopped for such decoking operation.
A variety of methods have been considered in order to overcome the disadvantages of coke build-up on reactor components, such as furnace tube inner surfaces. These approaches include: metallurgy upgrade to alloys with increased chromium content of the metal substrates used in the furnaces; adding additives such as sulfur, dimethyl sulfide (DMS), dimethyl disulfide (DMDS) or hydrogen sulfide to the feedstock; increasing steam dilution of feedstock, and improved process control; selectively pre-treating the inner surface of the coils; inert surface coating; and catalytic gasification of coke to produce CO/CO2 and hydrogen.
While some of the aforementioned methods and systems have general use in the petrochemical industry, it is desirable to provide a method and reactor that obviates and mitigates the shortcomings of the prior art and successfully reduces or eliminates the build-up of coke deposits.